Nonaqueous secondary batteries (for example, lithium ion secondary batteries; hereinafter simply referred to as batteries) have been put into practical use and widely used as secondary batteries for portable electronic devices. Further, in recent years, lithium secondary batteries have attracted attention not only as small batteries for portable electronic devices but also as large-capacity batteries for in-car use, power storage use, and the like. Therefore, regarding these batteries, demand for safety, lower manufacturing cost, longer life, and the like has been increased.
In general, a layered transition metal oxide represented by LiCoO2 is used as a cathode active material included in the nonaqueous secondary battery. However, the layered transition metal oxide is likely to undergo oxygen as generation at a relatively low temperature around 150° C. when the layered transition metal oxide is fully charged. The oxygen gas desorption accelerate a self-heating, which could cause a thermal runaway reaction in which oxygen gas is continuously generated. Accordingly, in a nonaqueous secondary battery having such a cathode active material, problems such as heat generation or ignition may occur.
In particular, high safety is required for large-sized and large-capacity nonaqueous secondary batteries for in-car use and power storage use. Therefore, it is expected that lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure, and the like will be used as the cathode active material because they have a stable structure and do not desorb oxygen under abnormal conditions.
In addition, a significant increase in the usage of cathode active materials is expected due to the wide use of nonaqueous secondary batteries for in-car use. Therefore, there is a problem of depletion of resources of elements included in cathode active materials. In particular, it is required that the use of cobalt (Co) as a resource be reduced due to its low crustal abundance. Therefore, it is expected that lithium nickelate (LiNiO2) and a solid solution thereof (Li(Co1-xNix)O2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4), and the like will be used as the cathode active material.
LiFePO4 has been widely studied from the viewpoints of improving safety and preventing depletion of resources. As a result of such studies, LiFePO4 has been put into practical use as a cathode active material through refinement of LiFePO4 particles, substitution of Fe and P with another element, and improvement of carbon coating on particle surfaces.
As compared to other cathode active materials, LiFePO4 has a problem of, for example, a low average potential value of 3.4 V. From the viewpoint of average potential, a cathode active material having an olivine structure with high potential such as LiMnPO4 has been studied. However, since LiMnPO4 has a lower conductivity than LiFePO4, it is known that Li intercalation/deintercalation is difficult (refer to PTL 1).
Therefore, substitution of a part of Mn with another element has been proposed to increase the charge-discharge capacity through improvement of charge-discharge characteristics (for example, refer to PTL 2). In addition, an active material represented by the formula AaMb(XY4)cZd (wherein A is an alkali metal, M is a transition metal, XY4 is PO4 or the like, and Z is OH or the like) has been proposed (for example, refer to PTL 3).
In addition, an active material represented by the formula LiMP1-xAxO4 (wherein M is a transition metal, A is an element having an oxidation number of +4 or less, and x is in a range of 0<x<1) in which the P site is substituted with A has been proposed (for example, refer to PTL 4).